This invention relates to a loudspeaker using a piezoelectric vibrator. When a piezoelectric vibrator is used as a driving source of an electro-mechanical or an electro-acoustic transducer, the efficiency of the transducer is much higher than that of other transducers such as the dynamic and electromagnetic types. For example, if we take a speaker as an electro-acoustic transducer, while the transducing efficiency of the usual dynamic type speaker is around 1% , that of a speaker using a piezoelectric vibrator is 15 to 20 times larger, i.e. 10 to 20%. Therefore, efforts have been made to apply a piezoelectric vibrator to an electro-acoustic transducer as a driving source. However, it is required that the speaker performance by such that the output vs. frequency characteristic be flat in a definite frequency range and this requirement is not satisfied by merely combining a piezoelectric vibrator with an electro-acoustic transducer. The reason is that the piezoelectric vibrator has a resonance at a particular frequency such that the output at that frequency becomes much larger than at other frequencies. Methods have been used to overcome this difficulty by controlling the voltage of the driving source of the piezoelectric vibrator or by wrapping the entire vibrator with a material having a large mechanical loss. However, this structure is too complicated and the effect unsatisfactory.